Steam sterilizer

ABSTRACT

In a steam sterilizer of the self-contained, single shell, steam generating type, the interior surface of the chamber is roughened to prevent the formation of water droplets, during cool-down, on all interior surfaces located above the load, to prevent water from dripping onto the load and thus to minimize the amount of moisture absorbed by the load during the sterilization process. In addition, a layer of insulation is located on the exterior surface of the upper area of the vessel so that, during cooldown, by forced air cooling of the vessel, the temperature of the upper area of the vessel remains sufficiently high to flash off condensate from the interior surface thereof.

United States Patent MacFarlane et a1.

[ STEAM STERILIZER [75] Inventors: Roderick T. MacFarlane, Pittsford;

Alfred C. Lawson, Fairport, both of [73] Assignee: Sybron Corporation, Rochester,

[22] Filed: Apr. 2, 1973 [21] Appl. N0.: 347,330

[52] U.S. Cl 21/56, 21/98, 165/105, 165/133 [51] Int. Cl A611 3/00, F28f13/18, F28f 19/06 [58] Field of Search 21/98, 94, 56; 165/105, 165/133, 135; 23/290 [56] References Cited UNITED STATES PATENTS 1,995,361 3/1935 Nagle 165/133 X 2,159,571 5/1939 Stack 165/135 X 2,532,655 12/1950 Backer 134/1 UX 2,648,108 8/1953 Pcntz 21/103 3,203,404 8/1965 Miller 165/133 X 3,206,381 9/1965 Neugebauer et al. 165/133 X 3,361,517 1/1968 Skaller 21/98 X 3,393,628 7/1968 Vischer, Jr.... 21/98 X 3,739,710 6/1973 Costa et a1 165/133 X FOREIGN PATENTS OR APPLICATIONS 449,440 6/1936 Great Britain 122/250 R OTHER PUBLICATIONS Fundamentals of Chem. Eng. Operations, by Larian Prentice-Hall, Inc. N.J., 1958, p. 86. Kents Mechanical Engineers Handbook Design and Jan. 21, 1975 Production Volume, John Wiley & Sons, NY. 1950 Section 24 pp. 27-32; Scientific Lib. Ref. TJ151.K4 V2 1950.

Evaporating, Condensing & Cooling Apparatus by E. Hausbrand Scott, Greenwood & Son, London, 1908, pp. 205-206, Sci. Lib.-TP363 H36VE.

Transport Phenomena by Bird; John Wiley & Sons Inc.; 1966; pp. 415-416.

Heat Transfer by Max Jakob; John Wiley & Sons Inc.; N.Y.; 1949; pp. 658-659; Lib.-QC320J3V.1. Elements of Heat Transfer by Max Jakob; John Wiley & Sons Inc.; 1957; PP- 204-205 Sci. Lib.-QC320J3E 1957.

Primary ExaminerJoseph Scovronek Assistant Examiner--Arnold Turk Attorney, Agent, or FirmSchovee & Boston [57] ABSTRACT In a steam sterilizer of the self-contained, single shell, steam generating type, the interior surface of the chamber is roughened to prevent the formation of water droplets, during cool-down, on all interior surfaces located above the load, to prevent water from dripping onto the load and thus to minimize the amount of moisture absorbed by the load during the sterilization process. In addition, a layer of insulation is located on the exterior surface of the upper area of the vessel so that, during cool-down, by forced air cooling of the vessel, the temperature of the upper area of the vessel remains sufficiently high to flash off condensate from the interior surface thereof.

14 Claims, 4 Drawing Figures PATENIEDJANZI I915 3861.873

FIG. 4

PRIOR ART STEAM STERILIZER BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to steam sterilization, and in a preferred embodiment, to minimizing moisture absorption by the load in a self-contained, single shell, steam generating type of sterilizer.

2. Description of the Prior Art In a steam sterilizer of a self-contained, steam generating type, the steam remaining in the chamber after sterilization, is dissipated prior to opening the door to the closed vessel. The method and apparatus described in the Skaller patent US. Pat. No. 3,361,517 minimizes many of the problems associated with rapid dissipation of the steam. However, this patented system does not solve the problem of the formation of water droplets at the top of the sterilizing chamber during cool-down. Should water droplets fall onto a wrapped package causing wet spots, air-borne and/or surface bacteria can easily penetrate such wet spots and re-contaminate the previously sterile contents. The above-mentioned Skaller patent employed an inner shell over the goods to prevent droplets from dropping onto the load. Regarding the sandblasted surface of the present invention, it is known to sandblast the interior of a jacketed rectangular sterilizing vessel to enhance the appearance of the welds.

It is a primary object of the present invention to provide an improved method and apparatus for overcoming the above-mentioned problem inherent in the prior art, without requiring the use of an inner liner over the load.

It is a specific object of the present invention to provide a method and apparatus for preventing the formation of droplets on the interior surface of the upper area (hereby defined as that area overlying the loadcontaining area of the sterilizing chamber), of the sterilizing vessel and the consequent falling of such droplets onto the load.

SUMMARY OF THE INVENTION The formation of water droplets on, and the consequent dropping of water droplets from, the interior surface of the upper area of a self-contained, single shell, steam sterilizer, are prevented according to the present invention as follows. The interior surface of at least the upper area of the sterilizing vessel is treated (for example, by sandblasting) to provide a surface on which the formation and development of droplets is prevented or reduced. The sandblasted surface preferably has a roughness of about 250-300 microinches. The roughness causes the condensate to form over a large area with a low profile configuration. This is in sharp contrast to the formation of heavy droplets with a low amount of contact area between the droplet and the vessel surface when a highly polished interior surface is used, due to the low surface tension between the condensate and the surface. The condensate is more readily flashed off of the surface, by whatever heat is available in the vessel wall, when there is a larger area of contact between the condensate and wall. Further, the interior surface of the upper area is kept free of irregularly shaped surface features, because droplets tend to form on such features.

In addition to the roughened interior surface, a layer of insulation is placed over only the upper area of the vessel, so that during cool-down, by forcing air across the outer surface of the vessel, the temperature of the upper area of the vessel remains high enough to flash off or rapidly evaporate condensate that develops thereon. Other means than insulation can be used to insure such flash off.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more fully understood by reference to the following detailed description thereof, when read in conjunction with the attached drawings, wherein like reference numerals refer to like elements, and wherein:

FIG. 1 is a schematic longitudinal cross-sectional view through a steam sterilizer made according to the present invention;

FIG. 2 is a schematic transverse cross-sectional view through the sterilizer of FIG. 1;

FIG. 3 is an enlarged fragmentary view through the upper area 42 of FIG. 2; and

FIG. 4 is an enlarged fragmentary view through an upper area of a prior art sterilizer vessel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT With reference now to the drawings, FIGS. l3 show a preferred construction of a steam sterilizer incorpo-,

rating the present invention, while FIG. 4 shows the formation of water droplets as occurs in prior art sterilizers. Reference will first be made to the general type of sterilizer to which this invention relates, a brief description of the prior art will then be made, and the present invention will then be described in detail.

FIGS. l-3 show a steam sterilizer 10 designed for operating automatic cycles to process such items, for example, as wrapped instruments, unwrapped instruments for immediate use, cotton dressings and bandages, disassembled syringes, liquids, and solutions. The system employed for the sterilization process is the admission of a controlled amount of distilled water into the chamber which is heated to provide steam. The steam rapidly and effectively penetrates the load. A fan cooling system rapidly cools the vessel 12 after the sterilizing phase, condensing the chamber steam and moisture in the load. The sterilizer 10 includes a cylindrical pressure vessel 12 (preferably of stainless steel) having a door 14 and enclosing a sterilizing chamber 16. The sterilizer 10 also includes a water supply tank 18 for filling a water reservoir 20 inside the chamber 16. A ter 2.21 29 1 ss. d atwattae 0 1 s p heater is located adjacent the bottom of the vessel 12. A fan 24 operated by a motor 26 (for example, a 1/100 H.P. 3,000 RPM motor) accelerates the condensation process during cool-down. The goods to be sterilized are placed in a plurality of trays 28 in the chamber 16. The preferred sterilizer 10 is a table top model that heats to about 280 F in about 24 minutes. A temperature of about 270 F is maintained for a minimum of about 3 minutes, and above 250 F for a minimum of about 8% minutes. The vessel 12 is cooled by the fan 24 to about F in about 17 minutes. The cycle time from a cold start is about 42 minutes.

The details and operation of a self-contained, steam generating type of sterilizer such as shown in US. Pat. No. 3,361,517 are well-known to those skilled in the art and therefore need not be described in detail here. The present invention, which is an improvement on such well-known type of sterilizers will now be described in detail.

Referring now briefly to the prior art shown in FIG. 4, it is a well-known problem of self-contained, single shell, steam generating sterilizing vessels that high interior surface condensation forms during the cool-down period of the cycle when the motor 26 is energized and rotates the fan 24 causing rapid condensation in the chamber 16. FIG. 4 shows the prior art wherein an interior surface 30 of the upper area of a sterilizer vessel 32 has a highly polished smooth surface on which condensate forms with very low surface tension between the condensate and the interior surface 30, into droplets 34 with a very low contact area 36 between the droplet and the surface 30. The latent heat that remains in the upper portion of the vessel 12 during cool-down is not sufficient to cause the droplet 34 to flash off of the surface 30. Further, water droplets tend to form from condensate at irregularities, such as openings and areas of non-uniform thicknesses, on the interior surface 30 of an upper area 42.

With reference now to FIGS. l-3 (particularly FIG. 3) this problem is solved, according to the present invention by: (1) providing an interior surface 40 of at least an upper area 42 of the vessel 12 (overlying the load-containing area of the sterilizer), with a roughened finish, and (2) insulating the exterior surface of the upper area 42 of the vessel 12. The surface 40 is roughened such that condensate 44 will not bead or form droplets that can drop onto the load, but rather the condensate will form over a large area of the surface 40 with a low profile configuration (in sharp contrast to the formation of the droplets 34). The roughness can be provided, for example, by sandblasting the metal surface 40. The roughness obtained depends, of course, on the grit used and can be conveniently expressed in microinches. A roughness of 250-300 microinches has been found to give superior results in preventing development of moisture droplets 34, while it has been found that a matte finish of 45 microinches will allow droplets 34 to from. The useful range of roughness is above 50 microinches and the preferred range is 250-300 microinches. Other methods of treating the interior surface 40 than sandblasting can, of course, be used to provide the rough surface. While the roughness is necessary only in the upper area 42, it can be employed over the entire inner surface of the chamber 16, if desired. For example, it may be easier to sandblast the entire inner surface than to restrict it to just one area thereof.

Regarding now insulating only the upper area 42 of the vessel 12, in the preferred embodiment a layer 46 of fiberglass insulation with a thin aluminum backing, three-quarter inches thick, is attached to the upper half of the vessel 12 by straps. The layer 46 of insulation extends over only the sides of the upper half of the cylindrical vessel 12 and does not cover the front or the backhead 52. The layer 46 of insulation provides a means for preventing rapid temperature decrease in the upper area 42 of the vessel 12, such as occurs in the remaining, uninsulated areas of the vessel 12. The use of the layer 46 of insulation on only the upper area 42 of the vessel 12 causes a temperature differential between the upper area 42 and the lower or remaining areas of the vessel 12, causing a more rapid heat loss (and therefore a more rapid temperature decrease) on the lower or remaining areas of the vessel 12 and greater maintainence of heat on the upper area 42 of the vessel 12 during cool-down. The upper area 42 is thus kept warmer and condensation forms and remains only on the remaining areas (i.e., the lower sides 50 and the backhead 52). The point is to provide a means for insuring that the temperature of the interior surface 40 of the upper area 42 of the vessel 12 is maintained sufficiently high to flash off the condensate that forms or develops thereon. The layer 46 of insulation maintains the temperature of the upper area 42, during cool-off. at a level sufficient to flash off (or rapidly evaporate) whatever condensate develops on the upper area 42 of the vessel 12. This aspect of the invention thus provides a flash zone at the top of'the chamber, and a condensing zone (not positioned over the load, thereby to return condensate to the water reservoir 20 without any dripping on the load). The condensing zone is cooled by exterior cooling means (such as air cooling, water cooling, or other means).

Other means than the layer 46 of insulation can be used, for this purpose, if desired. The term flash off is hereby defined for use in the present specification and claims to mean the instantaneous, nearly instantaneous, and very rapid evaporation or change of condensate from the liquid to the gas or vapor state. The term droplet is hereby defined for use in the present specification and claims as meaning condensate such as shown in FIG. 4 wherein the area of a horizontal crosssection at a mid-point is smaller than that at the largest part of the droplet.

Other means for preventing or reducing the formation of droplets 34 can be used such as other means for roughening a smooth surface, or even by forming the surface with a rough finish when it is initially made. The rough surface can be the inside surface of the vessel 12 as shown and described above or a liner with a rough surface can be inserted into the vessel. Alternatively, the interior surface can be coated with a material with a high affinity for water that will reduce the formation of droplets 34. Also, other means for maintaining the temperature of the upper area 42 can be used in place of the layer 46 of insulation, such as other types of insulation and/or a heater, and/or means for shielding the upper area 42 from the cooling effect of the forced air. Other means for cooling the vessel 12 can be used in place of the fan 24, such as, for example, water cooling.

The invention has been described in detail with particular reference to the preferred embodiment thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

I claim:

1. In a self-contained, single shell, steam generating sterilizer including a sterilizing vessel having a sterilizing chamber therein, and a fan adjacent the vessel for cooling the outside surface of the vessel, the improvement comprising:

a. the interior surface of an upper area of said vessel, overlying substantially all load-containing areas of said chamber, having a surface roughness greater than 50 microinches, said surface roughness being such that the formation thereon, from condensate, of water droplets is less than that which would occur under the same conditions on a surface having a roughness of less than 50 microinches; and

b. means for preventing during cool-down, rapid decrease of temperature of said interior surface of only said upper area of said vessel, whereby the temperature of said interior surface of only said upper area of said vessel is sufficiently high to flash off condensate therefrom.

2. The apparatus according to claim 1 wherein said surface roughness is between about 250 and 300 microinches.

3. The apparatus according to claim 1 wherein said preventing means comprises a layer of insulation on the outside surface of only said upper area of said vessel.

4. The apparatus according to claim 3 wherein said surface roughness is between about 250 and 300 microinches.

5. The apparatus according to claim 4 wherein said vessel is cylindrical with a horizontal axis.

6. In the method of reducing load absorption of water during sterilization of a load in a' self-contained, single shell, steam generating sterilizer including a sterilizing vessel having a sterilizing chamber therein and means for cooling the outside surface of the vessel during cool-down, the improvement comprising:

a. providing the interior surface of an upper area of said vessel, overlying substantially all loadcontaining areas of said chamber, with a surface roughness greater than 50 microinches and such that the formation thereon, from condensate, of water droplets, is less than that which would occur under the same conditions on a surface having a roughness of less than 50 microinches; and

b. preventing, during cool-down, rapid temperature decrease of the interior surface of only said upper area of said vessel, whereby the temperature of said interior surface of only said upper area of said vessel is sufficiently high to flash off condensate therefrom.

7. The method according to claim 6 wherein said providing step comprises providing said roughness in the range of about 250-300 microinches.

8. The method according to claim 6 wherein said preventing comprises providing a layer of insulation on the outside surface of only said upper area of said vessel.

9. The method according to claim 8 wherein said providing step comprises providing said roughness in the range of about 250-300 microinches.

10. A self-contained, single shell, steam generating sterilizer comprising a sterilizing vessel having a sterilizing chamber therein, including means for maintaining, during cool-down, the interior surface of only an upper area of said vessel, overlying substantially all load-containing areas of said chamber, at a temperature sufficiently high to flash off condensate therefrom.

11. The apparatus according to claim 10 wherein said vessel is cylindrical.

12. The apparatus according to claim 11 wherein said interior surface of only said upper area has a rough surface wherein the roughness is between about 250 and 300 microinches.

13. The method of preventing load absorption of water in a single shell, steam sterilizing vessel having a sterilizing chamber therein, during cool-down comprising maintaining the interior surface of only the upper area of the vessel, overlying substantially all loadcontaining areas of said chamber, at a temperature sufficiently high to flash off condensate therefrom.

14. The method according to claim 13 wherein cooldown is effected by a fan and wherein said maintaining step comprises providing a layer of insulation over only said upper area of said vessel. 

1. IN A SELF-CONTAINED, SINGLE SHELL, STEAM GENERATING STERILIZER INCLUDING A STERILIZING VESSEL HAVING A STERILIZING CHAMBER THEREIN, AND A FAN ADJACENT THE VESSEL FOR COOLING THE OUTSIDE SURFACE OF THE VESSEL, THE IMPROVEMENT COMPRISING: A. THE INTERIOR SURFACE OF AN UPPER AREA OF SAID VESSEL, OVERLYING SUBSTANTIALLY ALL LOAD-CONTAINING AREAS OF SAID CHAM ER, HAVING A SURFACE ROUGHNESS GREATER THAN 50 MICROINCHES, SAID SURFACE ROUGHNESS BEING SUCH THAT THE FORMATHAN THAT WHICH WOULD OCCUR UNDER THE SAME CONDITIONS ON A SURFACE HAVING A ROUGHNESS OF LESS THAN 50 MICROINCHES; AND B. MEANS FOR PREVENTING DURING COOL-DOWN, RAPID DECREASE B. MEANS FOR PREVENTING DURING COOL-DOWN, RAPID DECREASE OF TEMPERATURE OF SAID INTERIOR SURFACE OF ONLY SAID UPPER AREA OF SAID VESSEL, WHEREBY THE TEMPERATURE OF SAID INTERIOR SURFACE OF ONLY SAID UPPER AREA OF SAID VESSEL IS SUFFICIENTLY HIGH TO FLESH OFF CONDENSATE THEREFROM.
 2. The apparatus according to claim 1 wherein said surface roughness is between about 250 and 300 microinches.
 3. The apparatus according to claim 1 wherein said preventing means comprises a layer of insulation on the outside surface of only said upper area of said vessel.
 4. The apparatus according to claim 3 wherein said surface roughness is between about 250 and 300 microinches.
 5. The apparatus according to claim 4 wherein said vessel is cylindrical with a horizontal axis.
 6. In the method of reducing load absorption of water during sterilization of a load in a self-contained, single shell, steam generating sterilizer including a sterilizing vessel having a sterilizing chamber therein and means for cooling the outside surface of the vessel during cool-down, the improvement comprising: a. providing the interior surface of an upper area of said vessel, overlying substantially all load-containing areas of said chamber, with a surface roughness greater than 50 microinches and such that the formation thereon, from condensate, of water droplets, is less than that which would occur under the same conditions on a surface having a roughness of less than 50 microinches; and b. preventing, during cool-down, rapid temperature decrease of the interior surface of only said upper area of said vessel, whereby the temperature of said interior surface of only said upper area of said vessel is sufficiently high to flash off condensate therefrom.
 7. The method according to claim 6 wherein said providing step comprises providing said roughness in the range of about 250-300 microinches.
 8. The method according to claim 6 wherein said preventing comprises providing a layer of insulation on the outside surface of only said upper area of said vessel.
 9. The method according to claim 8 wherein said providing step comprises providing said roughness in the range of about 250-300 microinches.
 10. A self-contained, single shell, steam generating sterilizer comprising a sterilizing vessel having a sterilizing chamber therein, including means for maintaining, during cool-down, the interior surface of only an upper area of said vessel, overlying substantially all load-containing areas of said chamber, at a temperature sufficiently high to flash off condensate therefrom.
 11. The apparatus according to claim 10 wherein said vessel is cylindrical.
 12. The apparatus according to claim 11 wherein said interior surface of only said upper area has a rough surface wherein the roughness is between about 250 and 300 microinches.
 13. The method of preventing load absorption of water in a single shell, steam sterilizing vessel having a sterilizing chamber therein, during cool-down comprising maintaining the interior surface of only the upper area of the vessel, overlying substantially all load-containing areas of said chamber, at a temperature sufficiently high to flash off condensate therefrom.
 14. The method according to claim 13 wherein cool-down is effected by a fan and wherein said maintaining step comprises providing a layer of insulation over only said upper area of said vessel. 